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YMgNi4-based alloys exhibit reversible hydrogen absorption and desorption reactions at near room temperature. Here, we report that Co-substituted YMgNi4-based alloys exhibited higher hydrogen contents and lower hydrogen absorption and desorption reaction pressures than unsubstituted alloys. The effects of Co-substitution viewed from atomic arrangements were particularly clarified by synchrotron radiation powder X-ray diffraction, neutron diffraction, and inelastic neutron scattering. Powder neutron diffraction of the Co-substituted alloy at 5 MPa of D-2 pressure suggested the formation of gamma-phase deuteride (higher deuterium content) from beta-phase deuteride (lower deuterium content). However, no gamma-phase deuteride was observed in the unsubstituted alloys at 5 MPa. Therefore, the gamma-phase deuteride formation of the Co-substituted alloy at lower pressure led to higher hydrogen contents than the unsubstituted alloys. The combined results of powder neutron diffraction and inelastic neutron scattering suggested that the gamma-phase hydride of the Co-substituted alloy was continuously generated due to additional H atoms at the H atom sites in the beta-phase hydride because of the disordered H atomic arrangement involving H-H interactions. As a result, hydrogen absorption and desorption reaction pressures for the gamma-phase deuteride formation with higher hydrogen storage capacity were lowered.

Artículo escrito por un elevado número de autores, solo se referencian el que aparece en primer lugar, el nombre del grupo de colaboración, si le hubiere, y los autores pertenecientes a la UAM, Hydrides based on magnesium and intermetallic compounds provide a viable solution to the challenge of energy storage from renewable sources, thanks to their ability to absorb and desorb hydrogen in a reversible way with a proper tuning of pressure and temperature conditions. Therefore, they are expected to play an important role in the clean energy transition and in the deployment of hydrogen as an efficient energy vector. This review, by experts of Task 40 'Energy Storage and Conversion based on Hydrogen' of the Hydrogen Technology Collaboration Programme of the International Energy Agency, reports on the latest activities of the working group 'Magnesium- and Intermetallic alloys-based Hydrides for Energy Storage'. The following topics are covered by the review: multiscale modelling of hydrides and hydrogen sorption mechanisms; synthesis and processing techniques; catalysts for hydrogen sorption in Mg; Mg-based nanostructures and new compounds; hydrides based on intermetallic TiFe alloys, high entropy alloys, Laves phases, and Pd-containing alloys. Finally, an outlook is presented on current worldwide investments and future research directions for hydrogen-based energy storage

Hydrides are promising hydrogen storage materials owing to their higher gravimetric and volumetric hydrogen densities compared to compressed hydrogen gas and liquid hydrogen. Hydrogen is absorbed by different compounds (formation reaction of hydride) and thus exhibits different states—elemental hydrogen (H0), hydride ion (H−), and covalently bonded hydrogen (Hcov.)—in hydrides. The absorbed hydrogen is released as hydrogen gas (decomposition reaction of hydride). Therefore, it is important to understand hydride formation and decomposition based on the nature of chemical bonding of hydrogen (hydrogen states) in hydrides. Although it is difficult to directly observe hydrogen states, inelastic neutron scattering (INS), a powerful and useful technique, can be employed for this purpose as hydrogen vibrations in hydrides depend on its state. Herein, we provide an overview of INS studies of hydrogen vibrations in representative hydrides which are potential hydrogen storage materials. In particular, mode assignments focused on hydrogen states, local atomic arrangements around hydrogen atoms, hydrogen release reactions, and hydride formation processes based on observed hydrogen vibrations through INS are reviewed in this paper.

To synthesize a novel titanium-containing hydride, a Co3Ti alloy with a Cu3Au-type structure was hydrogenated at 9 GPa and 900 °C. Structural changes under high pressure were monitored via in situ synchrotron radiation x-ray diffraction at BL14B1, SPring-8. The in-situ measurement reveals that novel hydride Co3TiH~4 is formed at 9 GPa and 900 °C. When the sample was depressurized at room temperature, the formed hydride decomposed to (the previously reported) Co3TiH~1 at a pressure of ~1 GPa. Co3TiH~1 was recovered at ambient conditions and decomposed gradually into the Co3Ti alloy. Co3TiH~4 is thermodynamically stable only at pressures above 1 GPa.

The hydrogen storage properties and crystal structures of YMgNi4-based alloys, which were synthesized from (2 - x)YNi2 and xMgNi(2) (0.6

Neutron powder diffraction profiles were collected for iron deuteride (FeDx) while the temperature decreased from 1023 to 300 K for a pressure range of 4–6 gigapascal (GPa). The ε′ deuteride with a double hexagonal close-packed (dhcp) structure, which coexisted with other stable or metastable deutrides at each temperature and pressure condition, formed solid solutions with a composition of FeD0.68(1) at 673 K and 6.1 GPa and FeD0.74(1) at 603 K and 4.8 GPa. Upon stepwise cooling to 300 K, the D-content x increased to a stoichiometric value of 1.0 to form monodeuteride FeD1.0. In the dhcp FeD1.0 at 300 K and 4.2 GPa, dissolved D atoms fully occupied the octahedral interstitial sites, slightly displaced from the octahedral centers in the dhcp metal lattice, and the dhcp sequence of close-packed Fe planes contained hcp-stacking faults at 12%. Magnetic moments with 2.11 ± 0.06 μB/Fe-atom aligned ferromagnetically in parallel on the Fe planes.

Fe–Mo合金の6 GPa, 750℃における水素化反応の組成依存性を調べた。水素化反応は組成に応じて3種類に分類可能であることが明らかとなった。それぞれの水素化反応はFeに富む組成ではFe-H系の、Moに富む組成ではMo-H系の性質が表れやすく、中間の組成ではFe-HとMo-Hの中間の性質を示すことが明らかとなった。, Material Research Meeting 2021

Among the various functionalities of hydrides, their use in hydrogen storage has been the most intensively studied because hydrides can store hydrogen compactly and safely. Thus, hydrides are key materials for the hydrogen economy. Here, the hydrogen storage material Al3FeH3.9 has been synthesized from cost-effective earth-abundant metals, Fe and Al. Hydrides consisting of Al and transition metals with low hydrogen affinities are rare because such alloys are unstable. However, it is expected that appropriate mixing of the chemical states of hydrogen atoms would allow synthesis of Al-Fe hydrides. The experimentally determined crystal structure of Al3FeD3.9 suggests realization of the mixing of the chemical state of hydrogen. Al3FeH3.9 is more thermodynamically stable than AlH3, and it is likely that the mixing of the chemical state of hydrogen atoms is the source of increased stability. The results of this study confirm that by controlling the chemical states of hydrogen, it is possible to tune the thermodynamic stability of hydrides and thus realize novel functional hydrides.

The use of hydrogen in iron and steel has the potential to improve mechanical properties via altering the phase stability and dislocation behavior. When hydrogen is introduced under several gigapascals, a stoichiometric composition of hydrogen can be introduced for steel compositions. In this study, a face-centered cubic (fcc) stainless steel was hydrogenated under several gigapascals. When the steel was not hydrogenated, the microstructure after depressurization was an fcc with a hexagonal close-packed (hcp) structure. In contrast, the hydrogenation treatment resulted in a fine lath body-centered cubic (bcc) structure arising from diffusionless transformation. In particular, the bcc phase formed through the following transformation sequence: fcc → hcp → dhcp (double hexagonal close-packed phase) → bcc. That is, the use of hydrogenation treatment realized fine microstructure evolution through a new type of diffusionless transformation sequence, which is expected to be used in future alloy design strategies for developing high-strength steels.

The hydrogen desorption and absorption reactions of the complex metal hydride NaAlH4 are disproportionation processes, and the kinetics can be improved by adding a few mol% of Ti compounds, although the catalytic mechanism, including the location and state of Ti, remains unknown. In this study, we aimed to reveal the generating mechanism of catalytic Al–Ti alloy in NaAlH4 with TiCl3 using quantum multiprobe techniques such as neutron diffraction (ND), synchrotron X-ray diffraction (XRD), anomalous X-ray scattering (AXS), and X-ray absorption fine structure (XAFS). Rietveld refinements of the ND and XRD, profiles before the first desorption of NaAlD(H)4–0.02TiCl3 showed that Al in NaAlD(H)4 was partially substituted by Ti. On the other hand, Ti was not present in NaAlH4, and Al–Ti nanoparticles were detected in the XRD profile after the first re-absorption. This was consistent with the AXS and XAFS results. It is suggested that the substitution promotes the formation of a highly dispersed nanosized Al–Ti alloy during the first desorption process and that the effectiveness of TiCl3 as an additive can be attributed to the dispersion of Ti.

We report the lithium ionic conductivities of closo‑type complex hydrides synthesized from various molar ratios of lithium borohydride (LiBH4) and decaborane (B10H14) as starting materials. The prepared closo‑type complex hydrides comprised [B12H12]2−, [B11H11]2−, and [B10H10]2− complex anions. In addition, increasing the LiBH4 content in the starting materials increased the amounts of [B11H11]2− and [B10H10]2−, leading to an improved ion conductivity of the prepared sample. The present study offers useful insights into strategies for controlling the complex anion composition in emerging solid electrolytes of closo-type complex hydrides at the molecular level, and improving their ionic conductivities.

All-solid-state batteries incorporating lithium metal anode have the potential to address the energy density issues of conventional lithium-ion batteries that use flammable organic liquid electrolytes and low-capacity carbonaceous anodes. However, they suffer from high lithium ion transfer resistance, mainly due to the instability of the solid electrolytes against lithium metal, limiting their use in practical cells. Here, we report a complex hydride lithium superionic conductor, 0.7Li(CB9H10)–0.3Li(CB11H12), with excellent stability against lithium metal and a high conductivity of 6.7 × 10−3 S cm−1 at 25 °C. This complex hydride exhibits stable lithium plating/stripping reaction with negligible interfacial resistance (2500 Wh kg−1) at a high current density of 5016 mA g−1. The present study opens up an unexplored research area in the field of solid electrolyte materials, contributing to the development of high-energy-density batteries., All-solid-state batteries could deliver high energy densities without using organic liquid electrolytes. Here the authors report a complex hydride Li-ion conductor 0.7Li(CB9H10)–0.3Li(CB11H12) that exhibits impressive ionic conductivity and other electrochemical characteristics in an all-solid-state cell.

Geant4 Monte Carlo simulations were carried out to investigate the possible shielding materials of aluminum, polyethylene, hydrides, complex hydrides and composite materials for radiation protection in spacecraft by considering two physical parameters, stopping power and fragmentation cross section. The dose reduction with shielding materials was investigated for Fe ions with energies of 500 MeV/n, 1 GeV/n and 2 GeV/n which are around the peak of the GCR energy spectrum. Fe ions easily stop in materials such as polyethylene and hydrides as opposed to materials such as aluminum and complex hydrides including high Z metals with contain little or no hydrogen. Attenuation of the primary particles in the shielding and fragmentation into more lightly charged and therefore more penetrating secondary particles are competing factors: attenuation acts to reduce the dose behind shielding while fragmentation increases it. Among hydrogenous materials, 6Li10BH4 was one of the more effective shielding materials as a function of mass providing a 20% greater dose reduction compared to polyethylene. Composite materials such as carbon fiber reinforced plastic and SiC composite plastic offer 1.9 times the dose reduction compared to aluminum as well as high mechanical strength. Composite materials have been found to be promising for spacecraft shielding, where both mass and volume are constrained.

closo-type complex hydrides contain large cage-type complex polyanions in their crystal structures and thus can exhibit superior ion-conducting properties (e.g., Li and Na). However, the unique structures of complex polyanions have made it challenging to modify crystal structures, making systematic control of ion conductivity difficult. Here, we report an atom deficiency approach to enhance lithium-ion conductivity of complex hydrides. We find that lithium and hydrogen could be simultaneously extracted from Li2B12H12 by applying a small external energy, enabling the formation of atom deficiencies. These atom deficiencies lead to an increase in carrier concentration, improving lithium-ion conductivity by 3 orders of magnitude compared to that of a pristine material. An all-solid-state TiS2/Li battery employing atom-deficient Li2B12H12 as a solid electrolyte exhibits superior battery performance during repeated discharge–charge cycles. The current study suggests that the atom deficiency can be a useful stra...

A complex hydride (LaMg2NiH7) composed of La3+, two Mg2+, [NiH4]4– with a covalently bonded hydrogen, and three H– was formed from an intermetallic LaMg2Ni via an intermediate phase (LaMg2NiH4.6) composed of La, Mg, NiH2, NiH3 units, and H atoms at tetrahedral sites. The NiH2 and NiH3 units in LaMg2NiH4.6 were reported as precursors for [NiH4]4– in LaMg2NiH7 [Miwa et al. J. Phys. Chem. C 2016, 120, 5926–5931]. To further understand the hydrogen states in the precursors (the NiH2 and NiH3 units) and H atoms at the tetrahedral sites in the intermediate phase, LaMg2NiH4.6, we observed the hydrogen vibrations in LaMg2NiH4.6 and LaMg2NiH7 by using inelastic neutron scattering. A comparison of the hydrogen vibrations of the NiH2 and NiH3 units with that of [NiH4]4– shows that the librational modes of the NiH2 and NiH3 units were nonexistent; librational modes are characteristic modes for complex anions, such as [NiH4]4–. Furthermore, the hydrogen vibrations for the H atoms in the tetrahedral sites showed a narr...

難水素化金属から構成される合金の高圧水素化反応により、複数の新規水素化物が合成されているが、これらの水素化反応のメカニズムの解明は進んでいない。筆者らは難水素化金属から構成される合金の水素化反応メカニズム解明を目的として、Fe-Mo合金の水素化反応を合金組成を変えて調べた。水素化反応は組成により3種類に分類でき、構成元素単体の水素化物の性質と組成を考慮することでそれぞれの反応を説明できることを明らかにした。

Phase transformation in hydrogenated iron during depressurization from several gigapascals was investigated through in-situ synchrotron radiation X-ray diffraction and post-mortem electron backscatter diffraction measurements. The hydrogenated iron under 8.6 GPa at 293 K showed a double hexagonal close-packed (dhcp) structure, and it gradually transformed into a body-centered cubic (bcc) structure with decreasing pressure. The final crystal structure consisted entirely of a bcc phase. The structural change from dhcp to bcc structure was diffusionless-type phase transformation. The bcc phase showed lath morphology and could grow during aging under a constant pressure of 1.9 GPa, which indicated that it was bainitic-type transformation that required hydrogen diffusion or desorption.

Among the metal hydride materials, magnesium (Mg) and its alloys show excellent performance for hydrogen storage. The main drawback is the slow hydrogen absorption and desorption kinetics, the sole barrier to commercial adoption. In this work we use Mg thin films as model materials in order to study these kinetics, and observe the growth process of the hydride. Palladium (Pd) is used as a catalyst coating for improving the conditions of hydrogenation. The hydride formation is followed by in-situ X-ray diffraction. Microscopic imaging of the co-existence of Mg and MgH2 is presented. The microstructure change is clearly visible in the micrographs, despite the fact that sample preparation damages the hydride phase. The transformation from columnar grains of the as-deposited Mg thin film, to a grainy equi-axed structure film indicate that the hydride is observed. The hydride is immediately formed at the interface between the Pd and the Mg thin film and grows in a layer-like reaction towards the substrate (SiO2). These combined techniques provide an efficient methodology to follow the kinetics of hydride formation within the layer, and study further the diffusion coefficients and mechanism of hydrogenation.

Iron-containing complex hydrides are a fascinating class of materials for hydrogen storage applications because they consist of abundant iron and usually contains [FeH6]4− complexes, resulting in high hydrogen densities. In this study, we synthesized theoretically predicted Li3AlFeH8, which has the highest gravimetric hydrogen density of all transition metal complex hydrides, through a hydrogenation reaction of LiH, AlH3, and pure iron powder mixture under high pressure. The reaction process was observed in situ using a synchrotron radiation x-ray diffraction technique to clarify its reaction kinetics. The reaction temperature and pressure were changed to optimize reaction conditions for obtaining single phase Li3AlFeH8. Unfortunately, we did not obtain single phase Li3AlFeH8 because the reaction was slow. In addition, there were other phases with similar thermodynamic stabilities to that of Li3AlFeH8. Another novel hydride, LiAlFeH6, was found to be synthesized above 850 °C at 9 GPa.

The Li-5(BH4)(3)NH complex hydride, obtained by ball milling LiBH4 and Li2NH in various molar ratios, has been investigated. Using X-ray powder diffraction analysis the crystalline phase has been indexed with an orthorhombic unit cell with lattice parameters a = 10.2031(3), b = 11.5005(2), and c = 7.0474(2) angstrom at 77 degrees C. The crystal structure of Lis(BH4)(3)NH has been solved in space group Pnma, and refined coupling density functional theory (DFT) and synchrotron radiation X-ray powder diffraction data have been obtained for a 3LiBH(4):2Li(2)NH ball-milled and annealed sample. Solid-state nudear magnetic resonance measurements confirmed the chemical shifts calculated by DFT from the solved structure. The DFT calculations confirmed the ionic character of this lithium-rich compound. Each Li+ cation is coordinated by three BH4- and one NH2- anion in a tetrahedral configuration. The room temperature ionic conductivity of the new orthorhombic compound is close to10(-6) S/cm at room temperature, with activation energy of 0.73 eV.

Hexagonal close-packed iron hydride, hcp FeHx, is absent from the conventional phase diagram of the Fe–H system, although hcp metallic Fe exists stably over extensive temperature (T) and pressure (P) conditions, including those corresponding to the Earth’s inner core. In situ X-ray and neutron diffraction measurements at temperatures ranging from 298 to 1073 K and H pressures ranging from 4 to 7 GPa revealed that the hcp hydride was formed for FeHx compositions when x 3/H-atom, which was larger than that of the face-centered cubic (fcc) hydride. The hcp hydride showed an increase in x with T, whereas the fcc hydride showed a corresponding decrease. The present study provides guidance for further investigations of the Fe–H system over an extensive x–T–P region.

Ternary metal hydrides are convenient and valuable systems for investigating the metallization and superconductivity of metal hydrides because they can be synthesized under mild conditions and recovered under ambient pressure. In this study, the conducting behavior and structural phase transition of a hydrogen-rich metal hydride, $\mathrm{L}{\mathrm{i}}_{5}\mathrm{Mo}{\mathrm{H}}_{11}$, were investigated at pressures up to 210 GPa in a diamond anvil cell. The results showed that $\mathrm{L}{\mathrm{i}}_{5}\mathrm{Mo}{\mathrm{H}}_{11}$ transforms from an insulator to a poor metal at around 100 GPa. Superconductivity was observed at 100 GPa and retained until 210 GPa, and its maximum onset transition temperature was 6.5 K at 160 GPa. High-pressure synchrotron x-ray diffraction experiments revealed that the ambient-pressure hexagonal crystal structure is retained until at least 130 GPa. Furthermore, apart from the influence of pressure on the conducting behavior of $\mathrm{L}{\mathrm{i}}_{5}\mathrm{Mo}{\mathrm{H}}_{11}$, the effect of annealing time on the conducting and superconducting behaviors at room temperature and high pressure were also observed. We hypothesized that this time-dependent behavior is due to the restoration of the $\mathrm{Mo}{\mathrm{H}}_{9}$ cage structure after distortion or rotation caused by pressurization. These findings provide insight on the conducting and superconducting behaviors of ternary metal hydrides that, until recently, have been mostly studied by theoretical methods.

An intermediate phase LaMg2NiH4.6 found for the hydrogenation reaction of the ternary intermetallic compound LaMg2Ni to the complex transition-metal hydride LaMg2NiH7 has been investigated experimentally and theoretically. It is known that this reaction induces only minor rearrangement of the host metal atoms, and this feature is also observed for the intermediate compound. Another important feature is that the electronic structure of the intermediate phase is metallic, which is free from the energy barrier associated with the metal–insulator transition. The hydrogenation reaction to the intermediate phase proceeds even at room temperature, when the Ni–H clusters are formed. The internal bonding of the Ni–H clusters has a covalent nature, the same as those of NiH4 complexes in LaMg2NiH7. They most likely play as the precursor states for the following complex hydride formation. This picture is insightful to enhance the formation of complex hydrides.

Hydrogen release from Al-based complex hydrides composed of metal cation(s) and [AlH4]− was investigated using inelastic neutron scattering viewed from vibrational dynamics. The hydrogen release followed the softening of translational and [AlH4]− librational modes, which was enhanced by vibrational dynamics and the valence(s) of the metal cation(s).

Hydrogen-rich compounds show high- T c superconductivity related to dense hydrogen under extremely high pressure (above 100 GPa). A recent investigation to search for high- T c superconductive hydrides has advanced a synthesis technique using infrared laser heating of a hydrogen source material under conditions of extremely high pressure and temperature. In order to find a suitable synthesis route for high- T c superconductive hydrides, the selection of a hydrogen source material has to be considered. In this study, the synthesis of hydrogen-rich lanthanum hydrides (LaH x ) was performed using aluminium trihydride (AlH3) as the hydrogen source. Simple lanthanum on AlH3 was pressurized at 150 GPa and heated by infrared laser irradiation. After the laser heating, superconductivity at T c ∼ 70 K was observed at 170 GPa. This indicates that LaH x (x < 10) was synthesized using AlH3 as the hydrogen source under conditions of extremely high pressure.

The site occupancy of deuterium (D) atoms in face-centered-cubic nickel (fcc Ni) was measured along a cooling path from 1073 to 300 K at an initial pressure of 3.36 GPa via in situ neutron powder diffraction. Deuterium atoms predominantly occupy the octahedral (O) sites and slightly occupy the tetrahedral (T) sites of the fcc metal lattice. The O-site occupancy increases from 0.4 to 0.85 as the temperature is lowered from 1073 to 300 K. Meanwhile, the T-site occupancy remains ~0.02. The temperature-independent behavior of the T-site occupancy is unusual, and its process is not yet understood. From the linear relation between the expanded lattice volume and D content, a D-induced volume expansion of 2.09 (13) A3/D atom was obtained. This value is in agreement with the values of 2.14–2.2 A3/D-atom previously reported for Ni and Ni0·8Fe0.2 alloy.

Solid-state materials containing rotatable polyanions, such as B12 H122−, constitute a peculiar class of ionic conductors due to their unique transport behavior, where rotating polyanions promote phase transitions to disordered phases with several orders of magnitude enhancement in cation conductivities. A major drawback is the high temperature required to activate rotation and thereby low conductivities at room temperature. Here, we elucidate a mechanism to drastically reduce the temperature based on the use of pseudorotation in high-H coordination hydride complexes. We demonstrate this mechanism for an existing complex transition metal hydride Li5MoH11 containing MoH93−, and we present a strong potential of this material to unprecedentedly exhibit a high lithium ion conductivity of 7.9 × 10–2 S cm−1 at room temperature.

The hydrogen-release reaction of a complex transition metal hydride, LaMg2 NiH7 , composed of La3+ , 2×Mg2+ , [NiH4 ]4- and 3×H- , was studied by thermal analyses, powder X-ray, and neutron diffraction and inelastic neutron scattering. Upon heating, LaMg2 NiH7 released hydrogen at approximately 567 K and decomposed into LaH2-3 and Mg2 Ni. Before the reaction, covalently bound hydrogen (Hc °v. ) in [NiH4 ]4- exhibited a larger atomic displacement than H- , although a weakening of the chemical bonds around [NiH4 ]4- and H- was observed. These results indicate the precursor phenomenon of a hydrogen-release reaction, wherein there is a large atomic displacement of Hc °v. that induces the hydrogen-release reaction rather than H- . As an isothermal reaction, LaMg2 NiH7 formed LaMg2 NiH2.4 at 503 K in vacuum for 48 h, and LaMg2 NiH2.4 reacted with hydrogen to reform LaMg2 NiH7 at 473 K under 1 MPa of H2 gas pressure for 10 h. These results revealed that LaMg2 NiH7 exhibited partially reversible hydrogen-release and uptake reactions.

Aluminium-based complex hydrides (alanates) composed of metal cation(s) and complex anion(s), [AlH4]− or [AlH6]3− with covalent Al–H bonds, have attracted tremendous attention as hydrogen storage materials since the discovery of the reversible hydrogen desorption and absorption reactions on Ti-enhanced NaAlH4. In cases wherein alkaline-earth metals (M) are used as a metal cation, MAlH5 with corner-sharing AlH6 octahedron chains are known to form. The crystal structure of SrAlH5 has remained unsolved although two different results have been theoretically and experimentally proposed. Focusing on the corner-sharing AlH6 octahedron chains as a unique feature of the alkaline-earth metal, we here report the crystal structure of SrAlD5 investigated by synchrotron radiation powder X-ray and neutron diffraction. SrAlD5 was elucidated to adopt an orthorhombic unit cell with a = 4.6226(10) A, b = 12.6213(30) A and c = 5.0321(10) A in the space group Pbcm (No. 57) and Z = 4. The Al–D distances (1.77–1.81 A) in the corner-sharing AlD6 octahedra matched with those in the isolated [AlD6]3− although the D–Al–D angles in the penta-alanates are significantly more distorted than the isolated [AlD6]3−.

Despite many exploratory studies over the past several decades, the presently known transition metals that form homoleptic transition-metal hydride complexes are limited to the Groups 7-12. Here we present evidence for the formation of Mg3 CrH8 , containing the first Group 6 hydride complex [CrH7 ](5-) . Our theoretical calculations reveal that pentagonal-bipyramidal H coordination allows the formation of σ-bonds between H and Cr. The results are strongly supported by neutron diffraction and IR spectroscopic measurements. Given that the Group 3-5 elements favor ionic/metallic bonding with H, along with the current results, the true boundary for the formation of homoleptic transition-metal hydride complexes should be between Group 5 and 6. As the H coordination number generally tends to increase with decreasing atomic number of transition metals, the revised boundary suggests high potential for further discovery of hydrogen-rich materials that are of both technological and fundamental interest.